PROJECT SUMMARY Iron-sulfur (Fe-S) cofactor biogenesis is essential for most living organisms. The continuing long-term goal of this renewal proposal is to characterize protein-protein interactions (PPIs) critical for bacterial Fe-S cluster biogenesis by the Suf (sulfur formation) pathway, which is ubiquitous in prokaryotes including important pathogens. Due to the toxic nature of free iron and sulfide in cells, Fe-S cluster assembly and trafficking is highly choreographed by a complex network of protein partners. In Escherichia coli, Suf has evolved to act as an emergency pathway that is activated under conditions of oxidative stress or iron starvation. In other bacteria, such as Mycobacterium tuberculosis and Enterococcus faecalis, the Suf pathway is the sole source for Fe-S clusters, making it an essential pathway and potential target for antibiotic development. The Suf pathway consists of six proteins, SufABCDSE. SufS is a cysteine desulfurase that mobilizes persulfide (So) from L-cysteine. Persulfide generated by SufS must be transferred to the transpersulfurase protein, SufE, prior to delivery to SufB on the SufBC2D cluster scaffold. In vivo, Fe-S cluster assembly on the SufBC2D scaffold is dependent on the ATPase activity of SufC and acquisition of iron, which is likely regulated by SufD. Following cluster assembly, SufBC2D transfers the nascent cluster to SufA for downstream trafficking. While simple functional assignments are available for proteins in the Suf pathway, lack of detailed mechanistic descriptions prevents the rational design of small molecules to effectively disrupt the assembly process. During the previous funding period, regulation of the sulfur mobilization step was characterized in detail. The overall goal of this proposal is to investigate PPIs regulating cluster assembly and downstream trafficking. The hypothesis is that PPIs important for regulating cluster assembly and trafficking are governed by changes in the structure/dynamics of Suf proteins through the assembly process. We will test this hypothesis using a complementary, multi-pronged approach including hydrogen/deuterium exchange mass spectrometry (HDX-MS), protein crystallography, biophysical/biochemical characterization, and genetic complementation assays. To accomplish this broad approach, a research team with diverse backgrounds and a productive track-record has been assembled. The specific aims of this renewal proposal include: (1) characterization of Fe-S cluster-based regulation of sequential protein-protein interactions with SufBC2D, (2) determination of the role of ATP in the function of SufBC2D, and (3) identification of cluster trafficking interactions between the Suf system and the broader pool of Fe-S cluster carrier proteins. Completion of the experiments described in the proposal will provide a rigorous mechanistic description of how Fe-S cluster assembly and trafficking is regulated in the Suf pathway. These results can be leveraged to design potential antibiotics targeting bacterial Fe-S cluster pathways and may inspire novel therapeutic interventions for defects in analogous human pathways.